Falling is a significant problem in the care of the elderly that can lead to morbidity and mortality. From a physical perspective, falls cause injuries, while from the mental perspective, falls cause fear-of-falling, which in turn leads to social isolation and depression.
Fall detection systems are available that can provide an automated and reliable means for detecting when a user has fallen. If a fall is detected, the system issues an alarm which summons help to the user. This assures the user that adequate measures will be taken in the event that a fall occurs.
Commonly, fall detectors are based on an accelerometer (usually a 3D accelerometer that measures acceleration in three dimensions) that is part of a device that is to be attached to the user's body. The signals from the accelerometer are processed to determine whether a fall has taken place. This processing can be performed in the device itself or in an associated base unit.
The reliability of fall detection can be improved by making use of further sensors which can be used to detect various different features that are characteristic of a fall. Important features include the impact of the user with the ground during the fall (which can also be detected by the accelerometer) and an orientation change as the user falls (which can be detected by the accelerometer and/or by a magnetometer and/or a gyroscope).
As a fall can be defined as “an event that results in a person coming to rest unintentionally on the ground or other lower level”, another feature that can be detected is a change (primarily a reduction) in the height of the device above the ground. To this end, EP 1642248 proposes the use of an air pressure sensor to detect a change in the relative height measured by the device. Some currently available air pressure sensors make use of a membrane or diaphragm to measure the air pressure and are sensitive enough to be able to resolve changes in pressure equivalent to height changes of the order of 0.1 meter.
Typically, the air pressure P measured by a pressure sensor in a device is converted into altitude H according to
                              H          =                                    H              0                        +                          44330              ⁢                              (                                  1                  -                                                            (                                              P                                                  P                          0                                                                    )                                        0.19                                                  )                                                    ,                            (        1        )            where H0 and P0 are the altitude and the air pressure at sea level respectively. Two subsequent measurements are compared to determine the change in height of the device.
However, this conversion formula is a complicated formula to implement in an electronic device. The conversion typically has to be executed for every measurement sample, which puts a significant load on the consumption of battery power. Although it is possible to simplify this equation by approximating it into a polynomial, every measurement sample has to pass the conversion routine. In addition, the polynomial expansion is an accurate approximation at sea level, but it deviates when the sensor is operated at higher altitudes (for example more than 1000 m above sea level).
It is important to correct for these and other inaccuracies in the processing since the height changes that a fall detection or other activity algorithm attempt to monitor are close to the detection accuracy of the air pressure sensor (i.e. often around 0.5 meters, which occur when the sensor is worn at the wrist or waist, where the user is relatively short and/or when monitoring for falls, or just the opposite, for rises from a bed or chair).
Therefore, there is a need for an improved way to process the measurements from an air pressure sensor to determine the change in height of a device that avoids the inaccuracies present in the prior art methods.